A technique for providing a plurality of antennas both on the transmitting side and receiving side, preparing a plurality of radio wave channels in a space between the radio transmitting side and the radio receiving side and transmitting spatial multiplex signals through the channels. It is possible to improve transmission efficiency through MIMO.
There is a link adaptation technique as a peripheral technique of MIMO. Link adaptation refers to a technique of adaptively controlling the M-ary modulation number (transmission rate), coding rate and transmission power distribution according to variations in the channel environment between the transmitting side and the receiving side. When link adaptation is applied to MIMO and encoding is carried out and finished on a per stream (transmitting antennas for assigning data or beams) basis, it is possible to effectively utilize MIMO channels. What is referred to as a “stream” here may be referred to as a “codeword” as well. As such a technique, the technique disclosed in Patent Document 1 is known.
FIG. 1 is a block diagram showing a configuration of a MIMO system disclosed in Patent Document 1. In this MIMO system, channel estimator 21 of receiver 20 carries out channel estimation, and power and rate calculator 22 determines the rate and power of each stream using channel estimation values. Receiver 20 feeds back an indicator indicating the rate and power determined by power and rate calculator 22, to transmitter 10.
Transmitter 10 refers to the indicator fed back from receiver 20 and derives the rate and power applied to each stream. As a result of this, it is possible to set transmission rates and transmission power according to propagation conditions of each stream and realize high-speed transmission by keeping the reliability.
By the way, reference signals are orthogonally transmitted between transmitting antennas and interference between the transmitting antennas does not occur. Quality measurement for link adaptation is carried out using the reference signals.
On the other hand, high-speed data channels do not take orthogonal patterns between the transmitting antennas and signals of each transmitting antenna are mixed on the receiving side. For this reason, signals need to be demultiplexed on a per stream basis in a MIMO demodulating section.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-217752